1. Field of the Invention
The present invention relates to signal transmission and detection, and in particular, to techniques for compensating for signal distortions caused by signal dispersion and nonlinearities within the signal transmission media.
2. Description of the Related Art
Referring to FIG. 1, a conventional fiber optic signal system includes a data source 10, a light source (e.g., a laser) 12, the fiber optic medium 14, a signal detector (e.g., photodetector) 16 and an amplifier (e.g., transimpedance) 18, interconnected substantially as shown. The data source 10 provides a stream, or sequence, of data symbols 11 which modulate the light source 12 which, in turn, launches an optical signal 13 into the optical fiber 14. (Typically each data symbol consists of a single data bit.) At the reception end of the fiber 14, the optical signal 15 is received and detected by the detector 16, with the resultant signal 17 being amplified by the amplifier 18 to produce the electrical data signal 19 representing the sequence of data symbols. This signal 19 is then processed by clock and data recovery (“CDR”) circuitry (not shown) to recover the actual data and associated clock signals.
The detector 16 is typically some form of a direct detector, such as a photodetector. As is well known, the photodetector detects the modulated light forming the optical signal and, based on the amount of photonic energy in the optical signal, generates an electrical current signal corresponding to that photonic energy. Accordingly, the amplitude of the electrical current signal so generated varies in linear proportion to the received optical signal power since the amplitude of the current is proportional to the square of the optical signal amplitude.
It is well known that the bit rate of the data signal 11, as well as the length of the optical fiber 14, are limited in terms of how reliably a transmitted data can be received and accurately detected, due to the non-ideal characteristics of the fiber optic transmission medium 14. Referring to FIG. 2, for example, it is well known that an input data symbol 13, after propagating through the optical fiber 14, emerges as an optical signal 15 displaying a certain amount of signal dispersion. The amount of the signal dispersion increases in a manner corresponding to increases in the bit rate of the data signal 11 and length of the optical fiber 14.
One form of dispersion is chromatic dispersion which has a linear delay versus frequency characteristic. However, with direct optical signal detection, such as that done when using a photodetector, chromatic dispersion causes nonlinear distortions in the electrical signal of the receiver. Simple conventional linear equalization techniques are not adequate for compensating for such dispersion.
Referring to FIGS. 3A-3C, another form of dispersion is polar, or polar mode, dispersion. As shown in FIG. 3A, an optical signal transmitted through a single mode optical fiber actually transits light in two distinct polarization modes 21i, 21q. As is well known, the electrical fields of these two modes 21i, 21q are orthogonal to each other. As the optical signal travels through the optical fiber 14, these two signal modes 21i, 21q become misaligned, as shown in FIG. 3B. The amount of dispersion, or distance, 23a between these two modes 21i, 21q is dependent upon how asymmetrical certain characteristics of the optical fiber 14 are. For example, this dispersion 23 will increase in relation to the degree to which the refractive indices for each of the polarization modes 21i, 21q differ from each other within the optical fiber 14.
Referring to FIG. 3C, such asymmetrical characteristics of the optical fiber 14 tend to vary randomly along the fiber 14. Additionally, the optical signal can sometimes shift randomly between the polarization modes, thereby causing the phase shift between the two polarization modes to not accumulate consistently along the length of the optical fiber 14. Accordingly, the pulse duration 23b becomes stretched in time.
With polarization dispersion occurring in addition to chromatic dispersion, simple linear equalization techniques become even less effective as well as less practical due to the increasing complexity of the equalization circuitry necessary for compensation.
Referring to FIG. 4, the effect that such signal dispersion characteristics have upon the detected data signal can be better understood. As discussed above, the data signal consists of data symbols in the form of individual data bits. For this binary form of signal it is assumed that a binary value of unity (1) appears as a “high” signal value and a binary value of zero (0) appears as a “low” signal value at the output 17 of the detector 16 (or output 19 of the amplifier 18). However, consistent with the foregoing discussion, the dispersion effects of the optical fiber 14 are such that the value of the detected signal fails to achieve these ideal signal values due to the intersymbol interference (“ISI”) caused by the dispersion effects upon adjacent data bits as well as the present or desired data bit.
For example, let it be assumed that two adjacent data bits each have binary values of unity. Accordingly, this will produce the maximum signal value 24. Conversely, two adjacent data bits having binary values of 0 will produce the minimum signal value 26. Data bit pairs of “01” or “10” will produce signal values which are somewhere between these maximum 24 and minimum 26 values.
For example, following a bit value of unity, the signal value 28 will decrease and then either increase as value 28a or continue to decrease as value 28b when the value of the immediately subsequent data bit is unity or zero, respectively. Similarly, following a data bit value of zero, the signal value 30 will increase and then either continue to increase as value 30a or decrease as value 30b when the subsequent data bit has a value of unity or 0, respectively.
For purposes of this example, it is further assumed that the second bit of this bit pair is the transmitted bit intended for detection during the signal detection interval, centered about time Ts. By observing the signal at this time Ts, and comparing it to a threshold TH, a decision is made as to whether the signal level indicates a bit value of either unity or 0. However, as seen in FIG. 4, due to the dispersion effects and resulting ISI, there is a gap 34, referred to as the signal “eye”, between the possible signal values. As a result, incorrect decisions may be made as to the unity or zero value of the detected signal at time Ts.
Frequently, a fixed threshold value 32 is used for making this decision. The problem with this conventional approach, is that if the distortion affects cause the opening of the signal eye to not be centered about this threshold value 32 then the signal value will be incorrectly detected.
One conventional technique for compensating for this problem is to increase the effective size of the signal eye, thereby increasing the potential distance between detected signals representing values of unity and 0. Such technique uses a feedback signal to modify, e.g., increase or decrease as appropriate, the electrical signal 17/19 (FIG. 1) by shifting the signal wave for maximum 24 and minimum 26 levels up or down so that the effective threshold values 32a, 32b appear halfway between them. However, while this may be effective at low data rates, it becomes significantly less effective at high data rates.
Another conventional technique has been to modify the threshold, rather than modify the detected signal. With reference to FIG. 4, this would be done by shifting the threshold 32 in accordance with what the immediately preceding adjacent data bit value was. For example, if the immediately preceding adjacent data bit had a value of unity or zero, the effective threshold would be shifted to a higher 32a or lower 32b value, respectively.
While these techniques can be somewhat effective, such techniques do nothing to remove distortion from the data signal. Instead, such techniques merely use information about the distortion in an attempt to achieve an approximately equivalent, but inferior, effect.
Accordingly, it would be desirable to have a compensation technique for reducing ISI products by more directly compensating for the individual ISI products.